The present invention resides generally in the field of medical devices, and more particularly relates to artificial valve devices such as those implanted within the vascular system.
As further background, in all vertebrates, blood is transported away from the heart and throughout the body via arteries and returns to the heart through veins. To allow for optimal transport back to the heart and to reduce blood pressure build-up, veins contain various valves within their lumens, which open to permit blood flow to the heart but close to prevent the backflow of blood. Accordingly, venous valves function to provide the unidirectional flow of blood back to the heart.
Problems can arise when these venous valves fail to function properly. For example, venous valves can become incompetent or damaged by disease such that the backflow of blood is not prevented. When this occurs, blood pressure builds up and the veins and their valves become dilated, particularly in the lower extremities. If enough pressure builds up, the condition of venous insufficiency may develop. The severity of this condition is substantial, resulting in swelling, extensive pain, deformities, and, in the most severe cases, the development of ulcers can occur. If these ulcers become infected, amputation may ultimately be necessary to save the patient's life.
Currently, there is no proven cure for venous insufficiency. Basic treatments include elevation of the legs or the use of compression stockings. If surgery is determined to be necessary, vein stripping is typically performed, which involves the removal of the incompetent or damaged vein(s). Other surgical methods involve valvular reconstruction or transplantation.
Recently, the development of artificial and biological valves has been employed in an attempt to return normal pressure to the veins. There are a variety of these valves described in the art, which are generally designed to allow normal flow of blood back to the heart, while preventing retrograde flow. For example, U.S. Pat. No. 6,508,833 discloses a multiple-sided medical device comprising a closed frame of a single piece of wire or other resilient material and having a series of bends and interconnecting sides. The device has both a flat configuration and a second, folded configuration that comprises a self-expanding frame. The device is pushed from a delivery catheter into the lumen of a duct or vessel. A covering of fabric or other flexible material is sutured or attached to the frame to form an artificial valve. The flexible material utilized in the disclosed valves can be comprised of collagenous submucosa obtained from various animals, such as, for example, pigs, cattle, and sheep. This material can be processed and preserved so as to be capable of inducing host tissue proliferation, remodeling, and regeneration of appropriate tissue structures e.g., veins upon implantation in vivo (see, e.g., U.S. Pat. No. 6,485,723). The preparation of submucosal material is generally described in U.S. Pat. Nos. 4,902,508 and 5,554,389. The submucosal material can be prepared in large, flat sheets, which are subsequently cut and attached to a framing element, for example a stent, for deployment in a vein.
Despite work in the area there remain needs for medical products and methods for grafting within the vasculature, including the venous system, to improve blood flow. Desirably, such products and methods could eliminate or minimize the need for the presence of implanted support structures such as stents or frames, associated with the engrafted or implanted product. Such support structures commonly exert significant radial force upon vessel walls, and in certain situations may migrate deleteriously into the walls and/or undesirably reduce the compliancy of the vessel in which they are implanted. As well, such stent or frame structures can present increased risks for thrombosis or embolism.